We propose to investigate the structure of replicating P22 and lambda phage DNA by physical methods, primarily electron microscopy. Three projects are planned: 1. Determination of the relationship between the origin of phage P22 DNA replication and the unique starting site for DNA encapsulation. The methods combine partial-denaturation mapping with genetic techniques. 2. Development of methods, based on computer techniques, for construction of partial-denaturation maps of large DNA molecules such as bacterial episomes, chromosomes, or a chromosome from yeast. 3. Study of the mechanism of translocation by drug resistance elements.